Bone fastener and instrument for insertion thereof

ABSTRACT

A bone member fastener for closing a craniotomy includes a cap and a base interconnected by a narrow cylindrical collar. The cap has an externally threaded stud that screws into an internally threaded bore of the collar, thereby allowing the cap and base to be brought into clamping engagement against the internal and external faces of a bone plate and surrounding bone. In a particularly disclosed embodiment, the base of the fastener is placed below a craniotomy hole with the collar projecting into the hole, and the stud of the cap is screwed into the bore of the base from above the hole to clamp a bone flap against the surrounding cranium. This device provides a method of quickly and securely replacing a bone cover into a craniotomy. The distance between the cap and base can be selected by how far the threaded stud of the cap is advanced into the internally threaded collar. The fastener is therefore adaptable for use in several regions of the skull having various thicknesses. An insertion tool with a long handle permits safe and convenient placement of the base between the brain and the internal face of the bone plate. Some disclosed embodiments of the fastener have a cap and base that conform to the curved surface of the skull, for example by having an arcuate shape or flexible members that conform to the curvature of the bone plate and surrounding cranial bone as the fastener is tightened.

CROSS REFERENCE TO RELATED CASES

[0001] This case claims priority from U.S. patent application Ser. No.08/635,410 filed Apr. 26, 1996, which claims priority from U.S.Provisional Patent Application Serial No. 60/011,647 filed Feb. 14,1996, and U.S. Provisional Patent Application Serial No. 60/014,048filed on Mar. 25, 1996.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a fastener for securing a bone plate tosurrounding bone, for example when replacing a portion of the cranialvault removed during a craniotomy for a neurosurgical procedure. Theinvention also relates to an instrument for securing two threadedmembers to each other. The instrument may be used during neurosurgicalprocedures, or as a mechanical tool in many other contexts.

[0004] 2. General Discussion of the Background

[0005] A craniotomy is a surgical procedure in which a portion of thecranial vault is removed or folded back in a flap to permit surgicalaccess to the cranial contents (such as the brain). To perform thecraniotomy, several burr holes are drilled through the skull. The numberand position of these holes varies depending on the shape of bone to beremoved. For example, three burr holes are drilled at corner points if atriangular bone flap is desired. The burr holes are then connected byosteotomy cuts, for example using a Gigli flexible saw which is passedinternally between the burr holes. The saw is then oscillated back andforth to cut the skull along a line of separation (defined by theconnecting osteotomies) connecting adjoining burr holes. The bone coveris subsequently lifted off the underlying dura mater to expose thebrain. The bone cover may either be completely removed from the surgicalsite, or folded back in a flap along an uncut edge of the flap.

[0006] After completion of the operation, the bone cover must again befixed in its original position to protect the underlying brainConventional craniotomy closure requires that holes be drilled in thebone plate and surrounding cranium along the osteotomy lines, andstainless steel wire or silk sutures are then passed through the holesto retain the plate in place. Unfortunately, drilling the holes is timeconsuming and potentially dangerous, because of the risk of introducinginfection. The sutures are also unstable and prone to breakage. Sutureinstability or breakage can lead to dangerous movements of the cranialplate against the brain, with pathologic sequelae similar to a depressedskull fracture. The sutures closing the osteotomy lines are notaesthetically pleasing, because they can leave irregularities in theoverlying surface of the face or scalp. This irregularity isparticularly unsightly if the surface of the bone plate is not heldsubstantially co-planar with the surrounding bone.

[0007] Various fixtures have previously been proposed for securing thebone cover to the surrounding cranium. U.S. Pat. No. 5,201,737 disclosesa flexible plate having a plurality of vanes with holes for receivingbone screws. The plate is placed over a cranial burr hole and adjoiningosteotomy lines to provide external fixation of the bone cover to thesurrounding cranium.

[0008] Other external bone plates are shown in U.S. Pat. Nos. 4,651,724;4,923,471; 5,139,497 and 5,372,498. All of these plates are designed forexternal application to fractured bones, and require placement of aplurality of screws through the plates. As with the plate in U.S. Pat.No. 5,201,737 discussed above, placement of multiple screws through theplates is time consuming, predisposes to catastrophic infection, and isdifficult to remove once in place.

[0009] A variety of fixation devices are also known for fusing fracturedbones. An example of such a device is U.S. Pat. No. 2,511,051, in whichan externally threaded stud screws into an internally threaded shank.Movement of the stud into the shank is guided by an hexagonal wrenchthat is inserted through the shank into a countersunk receptacle on thetip of the threaded stud.

[0010] U.S. Pat. No. 3,875,936 shows an attachment for replacing atrochanteric head to the femur by providing a barbed shear washerbetween the femur and trochanteric head. U.S. Pat. No. 5,098,433 uses awinged compression bolt for fusing fractured bones. U.S. Pat. Nos.5,196,016 and 5,433,719 discloses fixation pins or screws for retainingbone fragments against one another.

[0011] In spite of the use of a variety of fasteners in orthopedic andneurosurgical procedures, improved techniques are still being sought tosecure a cranial cover to the surrounding cranium following acraniotomy. Improved methods and devices for securing the fasteners tothe skull are also disclosed.

[0012] Accordingly, it is an object of the present invention to providea fastener that is especially suitable for closure of craniotomies.

[0013] Yet another object is to provide such a fastener that can bequickly and efficiently installed, and which is capable of easy removalin the event that subsequent intracranial access is required for anotherneurosurgical procedure.

[0014] Yet another object of the invention is to provide such a fastenerthat avoids the aesthetic drawbacks of prior fasteners, such as largeindentations in skin overlying the craniotomy.

[0015] It is another object of the invention to provide such a fastener,and an instrument for manipulating the fastener, that allows it to beeasily inserted and removed, yet which provides a potentially permanentand reliable fixation of the cranial cover, thereby avoiding thepotentially catastrophic neurological consequences that can result fromdislodgement or depression of the cranial cover.

[0016] Finally, it is an object of the invention to provide aninstrument that is useful for engaging first and second members to eachother, particularly when the engagement must occur on opposite surfaces,and one of the surfaces is not easily accessible.

SUMMARY OF THE INVENTION

[0017] These and other objects are achieved by the bone cover fastenerof the present invention, which has internal and external fasteningmembers. At least one of the fastening members has a conforming surfacethat conforms to a curved surface, such as the inside surface of thecranial vault. In other embodiments, both the internal and externalfastening members have conforming surfaces, such that a surface of theinternal fastener conforms to the internal surface of the cranium, whilethe external fastener also has a surface that conforms to the externalsurface of the cranium.

[0018] In one embodiment, there is a connector on the internal fasteningmember and a connector on the external fastening member, and theconnectors cooperatively hold the internal and external fasteningmembers in a fixed relationship that fixes the bone cover in a defectfrom which the bone plate has been removed. The conforming surface maybe a curved surface of the internal fastening member that seats againstthe inner face of the cranium. In particularly disclosed embodiments,the internal and external fastening members are curved plates thatrespectively conform to the internal and external curvature of theskull. Alternatively, the conforming surface may be provided by flexiblestruts that deform into a curved configuration as the fastener istightened.

[0019] In other embodiments, a tab is provided on one of the fasteningmembers to prevent relative rotation between the fastening member andthe cranium. In particular, the tab is a relatively flat member thatextends away from the internal fastening member a sufficient distance tobe retained between the bone plate and the remaining cranium. The tab issufficiently thin that it fits within the gap between the bone plate andsurrounding cranium. When the external fastening member is rotatedrelative to the internal fastening members to interconnect threadedconnectors, rotation of the internal fastening member is opposed by thetab which is retained within the gap formed by the cranial osteotomyincision.

[0020] One of the connectors may be an elongated, externally threadedstud that projects from the first fastening member, while the secondconnector may be an elongated internally threaded collar that projectsfrom the second fastening member. The stud is rotationally threaded intothe collar, with the collar extending between the fastening members. Thecollar fits through the burr hole of a craniotomy incision, and eachfastening member is wider than the collar and the burr hole to provideclamping surfaces above and below the burr hole that engage the internaland external surfaces of the bone cover and surrounding cranium.Screwing the threaded stud into the internally threaded collar bringsthe opposing fastening members closer together, and tightens themagainst the internal and external surfaces of the bone cover andsurrounding cranium. The fastening members overlap margins of the burrhole and adjoining osteotomy lines to securely fix the bone cover to thesurrounding bone.

[0021] In a disclosed embodiment, one of the fastening members is a basewhich includes a disc having a flat inner face and a flat outer face. Aplurality of raised barbs extend from the inner face of the base toprovide frictional engagement between the disc and bone. The otherfastening member is a cap with an outer face that may be flat or convex,and a recess is provided in the outer face for engaging a drive memberthat rotates the cap. An externally threaded stud projects from an innerface of the base and an internally threaded collar projects from aninner face of the cap. The stud is approximately as long as the collar.The stud may be screwed partially or entirely into the collar, dependingon the thickness of the bone cover. Hence the fastener has theversatility to be used in different locations of the skull, where bonethickness varies.

[0022] The fastener is used in a method for fixing a bone plate, such asa cranial cover, in a bone defect, such as a craniotomy opening. Thebone plate has opposing internal and external surfaces that are to beheld in position substantially co-planar with internal and externalsurfaces of surrounding bone. The method provides the steps of placingthe base and cap of the fastening member on opposing internal andexternal surfaces of the bone plate, with the collar projecting into thehole, and a portion of the base and a portion of the cap overlapping theborder of the junction between the bone plate and surrounding bone. Thebase and cap are then rotated into threaded engagement with one another,until the cap and base tightly engage opposing surfaces of the boneplate and surrounding bone to clamp the bone plate in place. Inparticularly preferred embodiments, the fastener is placed through acraniotomy burr hole, with the cap and base covering and closing theburr hole.

[0023] In yet another embodiment of the invention, a fasteninginstrument is provided to secure first and second members to each other.The instrument includes a sleeve, a shaft or rod that slides within thesleeve, an attachment mechanism on the sleeve for engaging the firstmember to the instrument, and an engagement structure on the shaft thatengages the second member and pulls it into engagement with the firstmember. In more particular embodiments, the sleeve includes a handleextending out from the sleeve, and the rod includes a handle that allowsthe rod to be rotated and reciprocated relative to the sleeve. There isa threaded tip on the rod that engages an internally threaded collar ofthe first member to screw the first and second member to be moved intoengagement with each other.

[0024] In a more particular embodiment, the instrument includes a sleevethat has a handle (such as a rod or disc) extending from the sleeve. Arod slides within the sleeve, and has a threaded tip at a distal end ofthe rod, and a handle (such as a disc or curved hand grip) near aproximal end of the rod. The handle extends perpendicularly to the rod,and allows an operator to both rotate the rod and move the rod axiallywithin the sleeve. A distal tip of the sleeve that surrounds thethreaded rod has a plurality of locking members or prongs that extendfrom the tip of the sleeve and fit into complementary receptacles on oneof the fasteners to lock the fastener to the tip of the sleeve, forexample by frictional engagement of the prongs to one of the fasteners.

[0025] The instrument is designed to engage two threaded fasteners toeach other. The fastener includes a cap and a base, and a receptacle inthe cap that engages the prongs on the sleeve to lock the cap to thesleeve. The cap also includes an opening through which the rod canslide. The base of the fastener includes a post having both internal andexternal threads, and the threaded tip of the rod has threads that arecomplementary to the internal threads of the post. The cap has a collarwith internal threads that are complementary to the external threads onthe post.

[0026] The fasteners are attached to each other by first engaging thefastener cap to the sleeve of the base by introducing the prongs intothe receptacles on the cap. The rod is then extended through the cap,and the base secured to the rod by threading the threaded tip of the rodinto the internally threaded post of the base. The sleeve is thenaxially advanced over the rod until the external threads of the basepost abut the internal threads of the cap collar. The rod and sleeve arethen rotated relative to each other (for example by rotating the shaft)to screw the externally threaded tip of the post on the base into theinternally threaded collar of the cap. The threaded rod tip is thenunscrewed from the internally threaded post of the base. The connectedfasteners may then be disengaged from the instrument, for example bypulling the instrument away from the fasteners to disengage the lockingmember from the fasteners. Alternatively, axial movement of the rodtowards the fasteners brings the threaded rod tip into abutment againstthe internal threads of the post, and pushes the fasteners off thelocking members to disengage the fasteners from the instrument.

[0027] The instrument of the present invention can be used with manydifferent types of fasteners, in many situations where two fastenersmust be secured to each other in a difficult to reach location. Theinstrument is particularly useful when the fasteners are to be securedto opposite faces of a barrier that inhibits access to both faces of thebarrier. One particular fastener with which the instrument can be usedis the bone cover fastener of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic view of a portion of the human skull inwhich a craniotomy has been performed, and fasteners of the presentinvention have been placed to secure the cranial cover to surroundingcranium.

[0029]FIG. 2 is an enlarged, perspective view of one of the fasteners ofthe present invention, wherein the cap and base are disengaged.

[0030]FIG. 3 is a top view of the cap of the fastener of FIG. 2.

[0031]FIG. 4 is a top view of the base of the fastener of FIG. 2.

[0032]FIG. 5 is a cross-sectional view taken along section line 5-5 ofFIG. 1.

[0033]FIG. 6 is a cross-sectional view of the fastener shown in FIG. 2.

[0034]FIG. 7 is a cross-sectional view of a fastening instrumentsuitable for engaging another embodiment of the fasteners to oneanother.

[0035]FIG. 8 is an end elevational view of the fastening instrument ofFIG. 7.

[0036]FIG. 9 is a view of a fastener instrument, showing the fastenercap secured to the instrument.

[0037]FIG. 10 is a view similar to FIG. 9, but showing both the fastenercap and base secured to the instrument.

[0038]FIG. 11 is a view showing how the instrument is manipulated torotate the fastener base relative to the fastener cap to secure the baseand cap to each other.

[0039]FIG. 12 is a view showing the instrument disengaged from theassembled fastener.

[0040]FIG. 13 is a side elevational view showing another embodiment ofthe fastening instrument, in which the sleeve handle is a disc and therod handle is a curved handgrip.

[0041]FIG. 13A is an enlarged view of the tip of the instrument shown inFIG. 13.

[0042]FIG. 14 is a top plan view of another embodiment of the base,showing distribution of the barbs around the inner face of the base.

[0043]FIG. 15 is a side elevational view of the base shown in FIG. 14,illustrating the profile of the barbs.

[0044]FIG. 16 is a perspective view of the base of another embodiment ofthe fastener, in which an inner face of the base is convex and an outerface (not shown) of the base is concave.

[0045]FIG. 17 is a cross-sectional view of the assembled fastenerextending through a craniotomy burr hole and retaining a bone plate inplace within a cranial defect.

[0046]FIG. 18 is a perspective view of yet another embodiment of thebase having flexible struts.

[0047]FIG. 19 is a view of the base shown in FIG. 18, but wherein thestruts have flexed to conform to an interior curve of an internal faceof a bone plate.

[0048]FIG. 20 is a top view of the base shown in FIG. 18.

[0049]FIG. 21 is a view of another embodiment of the base which includesmultiple struts.

[0050]FIG. 22 is a top view of the base shown in FIG. 21.

[0051]FIG. 23 is a cross-sectional view of the fastener shown in FIGS.21 and 22, illustrating the fastening instrument that is used toassemble the fastener in the bone plate.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0052] The present invention includes fasteners for securing twotable-like or curved plate structures to each other. The invention alsoincludes a positioning instrument that is particularly useful forpositioning the fasteners in a surgical wound, and a fasteninginstrument for securing the fasteners to each other. This detaileddescription will describe several embodiments of the fasteners, as wellas an instrument that can engage the fasteners to each other. Theinstrument can be used to secure many different types of fasteners toeach other, but is shown in connection with the neurosurgical fastenersfor purposes of illustration.

[0053] A clamp 10 is shown in the drawings for closing a craniotomy(FIG. 1), which is a particular type of table-like structure. As isknown in the art, a craniotomy is performed by incising pericranium andmuscle with cutting diathermy in the line of an intended bone flap. Anincision is not made interiorly where a pedicle of pericranium ortemporalis muscle is often left uncut to allow some blood supply to thebone and overlying tissue to remain intact. Alternatively, thepericranium and muscle may be detached completely from the intended boneflap.

[0054] The craniotomy is performed by making a series of burr holes 12through the cranium about six or seven centimeters apart with aconventional trephine. The underlying dura mater is separated from thebone covering the osteotomy site (“the bone cover”) using a periostealelevator. Osteotomies 14 between the burr holes 12 are then made using aGigli flexible saw that is passed between adjacent burr holes and movedback and forth to make the osteotomies from the internal to the externalsurfaces of the cranium. The base of the flap, as illustrated in FIG. 1,is not made with a saw. A bone forceps (such as a de Vilbis forceps) isused to cut between burr holes until the bridge breaks at base 16 whenthe flap is elevated.

[0055] After placement of the burr holes and performance of theosteotomies, a bone plate 18 is separated from the surrounding craniumalong a line of separation formed by osteotomies 14. Each burr hole 12has a plate portion 12 a and a complementary cranial portion 12 b whichtogether form the completed hole 12. Trephines come in graduated sizes,for example between 0.5 and 2 inches (13-51 mm) diameter, for drillingburr holes of sizes varying across this usual range.

[0056] The bone plate 18 may be completely removed if osteotomiesconnect all of the burr holes 12. However, it is often preferred toleave an intact edge of the craniotomy (such as 16) to preserve theblood supply to the bone (as shown in FIG. 1). In either case, the boneplate or flap is referred to as the cranial cover.

[0057] After the neurosurgical procedure is performed, the bone defectmust be repaired by placing the cranial cover 18 back in the defect withopposing internal surface 20 and external surface 22 (FIG. 5)substantially co-planar or aligned with the internal surface 24 andexternal surface 26 of surrounding cranium 28 (FIG. 5). A transverseface 30 of bone cover 18 must also be fixed in apposition with atransverse face 32 of cranium 28, along a border of junction defined bythe osteotomy 14 between bone cover 18 and surrounding cranial bone 28.

Structure of the Fastener

[0058] In one embodiment, the fastener 10 is a one-piece member thatincludes a base 38 (FIGS. 2, 4-6) formed by a solid disc 40 having aflat inner face 42 and a flat outer face 44. Inner face 42 circumscribesan elongated, cylindrical collar 46 projecting outwardly from the centerof inner face 42. An internally threaded bore 48 extends longitudinallythrough collar 46, but does not extend through disc 40. Hence bore 48 inthis embodiment communicates only with a round opening 50 at the top ofcollar 46, and does not extend through disc 40, which has a flat, solid,lower face 44. In other embodiments, bore 48 may extend through thebase.

[0059] A plurality of raised, conical or pyramidal barbs 52 are integralwith and extend upwardly from inner face 42 of disc 40. As best shown inFIG. 4, four barbs 52 are located on face 42, equally spaced around disc40, and inwardly spaced from the perimeter of the disc. Barbs 52 taperto sharp tips 54 for engaging bone against which the barbs are placed,and into which the barbs may be embedded by pushing the barbs againstthe bone. The cancellous nature of the skull bone makes it particularlysuitable for embedment of the barbs therein.

[0060] A cap 60 fastens to base 38 to form a fastener that secures bonecover 18 in place, and occludes the burr holes. Cap 60 is a solid disc61 (FIGS. 5-6) with a convex outer face 62 that is smooth except for arecess 64 for engaging a complementary drive member (not shown) thatrotates cap 60. The illustrated recess 64 is hexagonal, and designed toreceive the tip of a surgical instrument resembling an Allen wrench.However, an elongated kerf could alternatively be scored in the surface62 for receiving the tip of an instrument resembling a screw driver.

[0061] Cap 60 has a flat, annular inner face 66 from which an externallythreaded stud 68 projects for inter-engagement with internally threadedbore 48 of collar 46. Stud 68 projects from the center of face 66, andthe external threads are helical and machined to complement and screwinto the internal helical threads of collar 46. Hence the collarresembles a nut into which the cap threads like a bolt. Stud 68preferably has a length (extending perpendicularly away from face 66along a longitudinal axis of fastener 10) that is approximately as longas threaded bore 48 of collar 46. The greatest width of cap 60 (asmeasured by the diameter of disc 61) and the greatest width of base 38(as measured by the diameter of disc 40) are each wider than the outerdiameter of cylindrical collar 46. The outer diameter of cylindricalcollar 46 is less than the diameter of burr holes 12, but the diametersof discs 40 and 61 are greater than the diameter of burr holes 12. Cap60 and base 38 each have a longitudinal axis of symmetry, such that thefastener when assembled is symmetric.

Method of Use to Close Craniotomy

[0062] The fastener 10 is used in a method of replacing a bone plate,such as cranial cover 18, following a craniotomy. As already described,the craniotomy is performed by providing a plurality of craniotomyholes, such as burr holes 12, through the skull. The burr holes aresubsequently connected by osteotomies 14 to create a separation borderfor the cranial bone cover (which may be a plate that is removed or aflap that is folded back along one edge). The craniotomy openingtherefore includes a portion 12 a of burr hole 12 formed in cranialcover 18, and a complementary portion 12 b of the hole formed insurrounding cranial bone 28.

[0063] Bone cover 18 has internal face 20 and external face 22 that arerespectively placed in substantially co-planar relationship withinternal face 24 and external face 26 of the surrounding cranial bone 28when a craniotomy is closed. Opposing transverse faces 30, 32 of thebone cover and surrounding bone appose along the separation border whenthe bone cover is in place. Faces 30, 32 are substantially parallel toeach other when in apposition.

[0064] In the method of the present invention, the bone cover 18 isfolded back along a base 16, or completely removed as a plate, to exposethe underlying dura mater and brain. Following the intracranialprocedure, and prior to replacing the bone flap into its originalorientation within the craniotomy opening, base 38 is placed below eachburr hole 12 with the inner face 42 of base 38 against the internalsurface 24 of cranium 28, such that one or more of barbs 52 engage theinternal face 24. Base 38 overlaps the margins of hole 12 and theseparation border between the cranial cover and surrounding bone (asshown in FIG. 1). Collar 46 projects upwardly into or through theportion 12 b of the hole, but preferably does not extend out of theexternal surface. Cranial cover 18 is then replaced into the craniotomyopening, for example, by folding the bone flap along base 16 back downinto the craniotomy opening.

[0065] Once cranial cover 18 is restored to its original position in thecraniotomy opening, complementary portions 12 a, 12 b now reform hole12. Collar 46 is centered in hole 12, with barbs 52 engaging theinternal faces 20 and 24 of cover 18 and cranium 28 (FIG. 5). Cap 60 isthen positioned over the reformed hole 12, with externally threaded stud68 above opening 50. Stud 68 is introduced into collar 46 throughopening 50, and a drive member (such as a surgical instrument resemblingan Allen wrench) is inserted into hexagonal recess 64 to rotate cap 60and advance stud 68 into threaded bore 48. Continued rotation of cap 60diminishes the distance between base 38 and cap 60, to tighten innerfaces 42, 66 of the base and cap against the internal and externalsurfaces of the cranium and cranial cover. The cap and base are therebybrought together in frictional engagement against opposing faces of thecranial cover 18 and surrounding bone 28, thereby clamping the cranialcover and surrounding bone to each other. Barbs 52 of base 38 anchor thebase to both the cranial cover and surrounding bone, preventing rotationof the base while the cap is screwed into the collar. The base isattached to the bone as barbs 52 become progressively advanced into thebone by tightening of the fastener.

[0066] The fastener is used to clamp the cranial cover to thesurrounding bone at one or more of the burr holes, and preferably all ofthe burr holes as shown in FIG. 1. A base 38 is therefore positionedbelow each burr hole 12 with collar 46 projecting up into the hole priorto replacement of cranial cover 18. A cap 60 is then screwed into eachinternally threaded collar until the cap and base clamp the bone cover18 and surrounding cranium securely to each other. The cap and base arein a fixed relationship determined by the degree of advancement of theexternally threaded stud 68 into the internally threaded collar 46, sothat movement of the bone plate is substantially prevented. The boneplate and surrounding bone are held firmly in place with respect to eachother, which avoids inadvertent depression of the cranial cover (withattendant catastrophic neurological consequences).

[0067] The smooth, convex top outer face 62 of cap 60 diminishes theaesthetic problem of visible indentations on the skull or face overlyingthe craniotomy burr holes. The closed base 44 tightly engages theinternal surfaces of the cranial cover and surrounding cranium,providing ideal occlusion of the hole, to help avoid infection ortrauma. The fastener also clamps the cranial cover in place quickly,thereby diminishing the period of time the brain must be exposed, andalso reduces medical expenses associated with prolonged time in theoperating room. The fastener is also easily removed, for example, byinserting an instrument into recess 64 of cap 60 and rotating the capoff of the fastener, to allow the bone flap to be subsequently removedif a follow-up neurosurgical procedure is required.

[0068] The fastener 10 is made of any biocompatible material, includingstainless steel, titanium alloy, polyglycolic acid, silicone rubber,teflon or nylon. Titanium is a particularly preferred material. Thebiocompatibility of these and other materials can be enhanced byfunctionalization of the surface of the fasteners. Plasma gas dischargeand corona treatment with reactive groups introduced on polymericsurfaces have been described as ways to modify biomaterial surfaces. SeeLee et al., Biomaterials 12:443, 1991 and Lee et al., J ColloidInterface Sci. 151:563, 1992.

[0069] The fastener 10 may be made in many different sizes. For purposesof illustration, the diameter of the stud 68 can be 7 mm (the samediameter as cylindrical bore 48); the external diameter of collar 46 maybe 8 mm; the diameter of disc 61 can be 20 mm; the diameter of disc 40may be 20 mm. In one preferred embodiment, stud 68 is 6 mm long, whichis the same length as collar 46 and its internal bore 48.

[0070] The fastener of the present invention is adjustable for use inrepairing craniotomies through bone of varying thickness. The pterion(in the temporo-parietal region) for example, is quite thin and mayrequire a fastener with a 3 mm stud and collar, with advancement of thecap and base toward each other until the stud 68 has been completelyadvanced into collar 46 and face 66 of disc 61 abuts against collar 46.The 3 mm stud and collar, however, would also allow the fastener toclamp bones up to 6 mm thick (if the stud is only initially advancedinto the collar). The parietal or frontal bone, however, may be 10-20 mmthick. A fastener with a 6 mm long stud 68 (and a 6 mm long internallythreaded collar) can clamp bones together varying in thickness from 6 mmto 12 mm. A longer stud and collar (for example the stud and collar each12 mm long) could clamp together bones from about 12 mm thick (when thefastener is fully tightened) to 24 mm thick (when the cap and base areinitially engaged but before more rotation of the cap into the baseoccurs).

[0071] Other variations of the disclosed fastener are possible. Theinternally helically threaded collar, for example, may depend from thecap while the externally helically threaded stud may project from thebase. The cap and base may assume many shapes other than circular, andmay for example be square or triangular. Bone plates other than in theskull may be secured to surrounding bone, for example a relatively flattop bone such as the trapezium, mandible, maxilla, or bones of theorbit. The convex face of cap 60 may also be flat, particularly in areasof the skull (such as the temporal bone) with relatively flat externalsurfaces.

[0072] The fastener may also be used to fix adjacent members together,even where the members are curved. However, the embodiment of thefastener shown in FIGS. 16-23 is preferred for curved surfaces, and willbe subsequently described.

Fastening Instrument

[0073] An instrument 100 for securing fastener members to each other isshown in FIGS. 7-12 to include an elongated tubular sleeve 102 (seeespecially FIG. 7) having a proximal end 104 (nearer the surgeon orother user) and a distal end 106 (farther away from the surgeon or otheruser). Two tubular handle members 108, 110 extend diametrically awayfrom each other, perpendicular to the axis of sleeve 102. Distal end 106has two small posts 112, 114 (FIGS. 7 and 8) projecting 1-3 mm (forexample 2-3 mm) away from the flat surface 116 of distal end 106. Posts112, 114 are spaced approximately 180° from each other, and serve asprongs that fasten the fastener to the instrument.

[0074] An elongated rod 120 (FIG. 7) slides within sleeve 102. A tubularguide member 107 may be placed in the sleeve 102 to fill empty space andinhibit relative transverse movement between the sleeve 102 and rod 120that freely slides axially in the sleeve. Rod 120 includes a proximalend 122 and a distal end 124. A disc 126 is fixed to the proximal end122 of rod 120 by hex nut 127, and extends perpendicularly to thelongitudinal axis of rod 120. Disc 126 provides a handle on theinstrument that can be grasped by a user to reciprocate or rotate rod120 within sleeve 102.

[0075] The distal end 124 of rod 120 has an externally threaded,cylindrical, reduced diameter externally threaded tip 128. In oneembodiment, sleeve 102 is approximately 14 cm long, with the span ofarms 108, 110 being approximately 9 cm. Rod 120 is longer than sleeve102, with a total length of approximately 16 mm from tip 128 to disc126. The disc 126 is a solid, cylindrical disc having a diameter ofapproximately 3½ cm. Dimensions of the instrument may vary.

[0076] Instrument 100 is used to engage a first and second fastener toeach other. In this disclosed embodiment, the first fastener is a cap130, which is in the shape of a disc. Two openings 132, 134 through thedisk are complementary to the size and location of posts 112, 114 onsleeve 102. Hence openings 132, 134 provide receptacles into which posts112, 114 can be inserted and snugly received to secure or lock cap 130to instrument 100, for example by frictional engagement between theposts 112, 114 and the receptacles in the cap 130.

[0077] A central opening 136 is provided through the center of the cap,along the axis of rotation of cap 130. Central opening 136 has adiameter only slightly greater than the diameter of rod 120, such thatthe rod can slide through and be guided by the walls of central opening136. An internally threaded collar 138 (having helical internal threads)projects downwardly from cap 130 around central opening 136, and thecentral opening 136 communicates with and can extend through the collar138.

[0078] The second fastener member is a base 140 which has the shape of aplate or disc with a central stud 142 projecting upwardly therefrom.Stud 142 is provided with external helical threads 144 that arecomplementary with the helical internal threads of collar 138, such thatcollar 138 can be rotated to thread cap 130 onto base 140.

[0079] Three sharp projections 150, 152, 154 extend upwardly from thedisc of base 140 to help lock the base against a bearing surface. Theseprojections 150-154 are located at the periphery of base 140, and areequally spaced about 60° from one another (such that all of theprojections are on one-half of the base, preferably all within 120° ofeach other). The projections are substantially triangular, and each hasa sharp apex that is suitable for embedment in a bearing surface (suchas the internal surface of the skull).

[0080] Use of the instrument 100 to rotate the cap and base relative toone another, for engaging them to each other, is demonstrated in FIGS.9-12. FIG. 9 shows the first step, in which rod 120 is inserted throughopening 136 of cap 130 until locking posts 112, 114 are inserted intoopenings 132, 134 and cap 130 is secured in place on end 106 of sleeve102. The frictional engagement between locking posts 112, 114 and thereceptacles formed by openings 132, 134 holds cap 130 on end 106.

[0081] As shown in FIG. 10, base 140 is then attached to rod 120 byinserting threaded tip 128 (FIG. 9) into internally threaded stud 142 ofbase 140 (while the base is secured in place against an inner face ofthe bone plate). Rod 120 is then rotated in the direction of arrow 146(FIGS. 8 and 10) to screw threaded tip 128 into internally threaded stud142 of the base. Rod 120 is rotated by turning disc 126 until base 140is tightly secured on the tip. The step of threading base 140 onto tip128 is preferably performed with rod 120 fully extended (as shown inFIG. 10) by exerting axial pressure against disc 126.

[0082] Rod 120 is then axially retracted relative to sleeve 102 (orsleeve 102 is advanced relative to rod 120) (FIG. 11) to introduceexternal helical threads 144 on the stud 142 into the internal helicalthreads in 138 of cap 130. With the rod in the fully retracted position,sleeve 102 is rotated by turning handle 110 in the rotational directionshown by arrow 146 (FIG. 11). Base 140 will remain stationary, becausebarbed projections 150-154 on the base engage bone, and inhibit rotationof the base. Hence rotation of sleeve 102 in the direction shown byarrow 146 screws internally threaded collar 138 on to externallythreaded stud 142.

[0083] After collar 138 has been fully screwed on to base 140, dischandle 126 is then rotated in the direction of arrow 147 (the oppositedirection of arrow 146) to unscrew threaded tip 128 of the shaft 120from the internal threads of stud 142. When tip 128 is fully unscrewedfrom stud 142 and collar 138, the tip is axially retracted from opening136 to fully disengage the instrument 100 from the fastener. When usedin a surgical procedure, the instrument may then be withdrawn from thesurgical wound, leaving the fastener firmly in place engaging theopposing faces of the bone plate and surrounding bone.

[0084] The fastening instrument 100 can also be used in non-surgicalapplications, for example securing a fastener to opposing faces of awork piece such as a wall or adjacent pieces of wood. Variations of themethod of using the instrument can also be used, in both surgical andnon-surgical applications. For example, the base 140 can be rotated toscrew it on to threaded tip 128 instead of rotating threaded tip 128 toscrew it into the base. Only relative rotation between the base andsleeve is needed. Similarly, the base collar can be screwed into the capcollar by providing relative rotation between the cap and base, withoutnecessarily rotating the cap by turning the sleeve 102. The rod caninstead be rotated (in a direction 147) to screw the base into the capby relative rotation between the base and cap.

[0085] The relationships of the various parts of the instrument to thefastener are shown in the drawings, with particular reference to FIG.11, wherein rod 120 has been axially retracted and cap 130 rotatedrelative to base 140 to attach the cap and base by threaded engagementof the helical threads. Base disc 140 has a lesser diameter than cap130. In the disclosed embodiment, base disc 140 has a diameter of about11 mm, while cap 130 has a diameter of about 13 mm. Stud 142 has aheight of about 9 mm, while the height of collar 138 is about 4 mm. Thesmaller diameter of base disc 140 helps the base fit better against theinterior face of the cranial vault, which has a steeper curvature thanthe curvature of the outer cranial cover.

[0086] The barbs 150, 152 and 154 are all within a 120° sector of basedisc 140. This allows the barbs to be placed on the intact portion ofthe skull bone, without being embedded into the removable portion of thecranial cover. Hence the cranial cover can be removed when desired bydisengaging the cap and base, without disengaging the barbs from thebone of the plate that is to be removed.

Alternative Embodiment of Fastener Instrument

[0087] Another embodiment of the fastener instrument is shown in FIGS.13-13A, and is designated instrument 200. This instrument is similar toinstrument 100 shown in FIGS. 7-12, hence like parts have been givenlike reference numerals plus 100 to denote the similar parts, beginningwith instrument 200. This embodiment differs, however, in that the rodhandle 226 is not a disc, but is instead a handle grip with an arcuatefrusto-hemispherical outer face. Alternatively, the handle 226 may behemispherical or spherical in shape. The advantage of this shape is thatit presents an arcuate side surface that can be easily grasped by thehand, with the fingers directed somewhat inwardly (toward thelongitudinal axis of the instrument 200). Handle 226 provides a securehandgrip that is also easily rotated during use of the instrument.

[0088] The sleeve handle 210 also differs from the perpendicular rods108, 110 that served as the sleeve handle in instrument 100. The sleevehandle is instead a disc that circumscribes the sleeve 202 and extendsperpendicularly outwardly therefrom. The circumferential face 211 ofdisc 210 is serrated or otherwise roughened to increase frictionalengagement between face 211 and the hand of a surgeon or other user ofthe instrument. Improved frictional engagement between the handle andhand is particularly helpful, for example, in surgical procedures duringwhich blood or other body fluids may moisten the handle and make itslippery.

[0089] The friction engaging posts 212, 214 have also been elongated to2-3 mm (for example 3 mm) to provide prongs that more securely hold thefastener cap on the instrument. The receptacle holes have beencorrespondingly elongated, and tightly engage the posts 212, 214 toselectively hold the instrument and fastener together. FIG. 13A alsoshows an alternative embodiment of the posts or prongs in which acircular coil spring or O-ring 215 (shown in place on post 212) isseated in an annular indentation (shown as 217) on each post. The coilspring is somewhat resilient, such that the spring is slightlycompressed as posts 212, 214 are introduced into the receptacles of thecap. These compression springs provide improved engagement of the poststo the cap, such that the cap is even less likely to be dislodged fromthe instrument during use. Secure engagement of the cap to theinstrument is important to allow the instrument to be manipulated, ifdesired, with one hand.

[0090] Another embodiment of the fastener base is shown in FIGS. 14 and15. The base 340 is shown with six barbs 350, 352, 354, 356, 358 and 360distributed equally around the periphery of and extending from the innerface 341 of base 340. The barbs have a sharp profile, for example theapex of the barb forms and angle α that is about 30 degrees or less(FIG. 15). Each barb extends 2-3 mm in height above the surface of innerface 341. This height has been found to allow the barbs to enter bone(such as a bone plate) to a depth that provides particularly superiorengagement between the fastener base and bone plate. Distribution of thebarbs around the base (instead of placing the barbs in only a sector ofthe base, for example a 60° or 120° sector) allows the barbs to engagethe bone plate even if the base rotates slightly from its originalposition of placement.

Alternative Fastener Embodiments

[0091] Other embodiments of the fastener are contemplated as part of thepresent invention, and shown in FIGS. 16-23. A specific embodiment ofthe fastener 410, for example, is shown in FIGS. 16 and 17. Thatembodiment is similar to the fastener embodiment of FIGS. 5 and 6,except that the cap and base are curved to conform to the shape of theexternal and internal surface of the cranial vault. The range ofcurvatures of the human skull may range, for example, from a radius ofcurvature of 65-105 mm, and the radius of curvature of the cap and basecan extend across this same range.

[0092] Fastener 410 includes a base 438 and a cap 460 made ofpolyethylene, and each having a surface with a radius of curvature of 84mm that fits against the skull. The base (which is illustrated inisolation in FIG. 16) has a member 438 which is capable of conforming tothe shape of a curved surface against which the member seats. In thedisclosed embodiment of FIGS. 16-17, member 438 has a curved uppersurface 440 which has a shape defined by a portion of a sphere. Thecurvature of surface 440 is substantially the same as a curvature of theinternal surface of the bone plate 18 and surrounding bone 28 that formthe internal face of the cranial vault. Bottom surface 444 of member 438is also curved (and preferably parallel to curved surface 440) so thatboth the top and bottom surface of the base member 440 mimic the curveof the surrounding cranium. This curvature of the base helps member 438fit against the internal face of the cranial vault, and avoids damage tosurrounding dura mater that can occur when a flat plate fastener basedoes not conform well to the curvature of the skull.

[0093] In the disclosed embodiment, top member 461 of cap 460 is alsocurved to conform to the external curvature of the skull, such that cap460 fits unobtrusively against the external surface of the skull. Thecurved cap is less obtrusive than a flate plate, and creates less of apost-surgical cosmetic defect in the overlying scalp. The curved platecreates less of a pucker in the overlying tissue.

[0094] Another feature of the fastener 410 is that it does not requirebarbs extending up from the base member 438 into bone to preventrelative rotation between base 410 and the bone against which the baseseats. Instead, base 438 includes an upwardly extending tab 453 thatextends into the osteotomy incision 14 between burr holes to opposerotation of base 438 as cap 460 is screwed into it. Tab 453 is aprojection that extends a sufficient distance from base 438 to beretained within the osteotomy incision. For example, the tab extendsabout at least one half the height of collar 446, or to a height thesame height as the collar, or even slightly higher than the collar (asshown in FIG. 17). The range of tab heights, for example, may be 0.5 to11.5 mm. Tab 453 is preferably a thin plate-like member that issubstantially flat (with a thickness no greater than the osteotomyincision), for example, about 1-2 mm thick, or 1 mm thick, and extendsradially from collar 446 substantially the entire radial width of base438, and has a top that is not sufficiently sharp to penetrate bone. Theillustrated tab 453 has a curved, blunt top 455.

[0095] Assembly of fastener 410 is illustrated in FIG. 17, which issimilar to FIG. 5, and wherein like parts have been given like referencenumerals plus 400, and the fastener 410 is positioned in a burr holewithin an osteotomy separation. Base 438 can be positioned against theinternal surface of bones 18, 28 such that the curved member 438conforms to the shape of the skull, and tab 453 projects up into theosteotomy separation between bone plate 18 and surrounding bone 28. Base438 can be placed in this position, for example, using the instrumentshown in FIGS. 7-10 or FIGS. 13-13A.

[0096] Once the base 438 is appropriately positioned, cap 460 is thenplaced above the base, and externally threaded stud 468 is threaded intointernally threaded collar 446 by rotating cap 460. Rotation of base 438does not occur as cap 460 is rotated, because tab 453 is retained withinthe osteotomy incision and abuts against either the bone plate 18 orsurrounding bone 28. Stud 468 is rotated into collar 446 until cap 460and base 438 are appropriately tightened against the bone.

[0097] In an alternative embodiment of FIG. 16, the tab 453 can extendfrom collar 446 instead of surface 440. The base includes both surface440 and collar 446.

[0098] Another embodiment of the fastener is shown in FIGS. 18-20, whichshow a strut embodiment of the base. The base 538 is made of fourflexible strut members 541, 543, 545 and 547 that cooperatively form an“X” shaped base. Each strut is a thin, rectangular extension that ismade of plastic (such as high density polyethylene) or other flexiblematerial, and which can be flexed within about 2-15°, preferably atleast 2°, of its resting position (as shown by α in FIG. 19) withoutbreaking, and in the disclosed example flexes about 5°. Internallythreaded collar 546 (having threaded bore 550 shown in FIG. 18) projectsupwardly from the intersection of the struts, and is connected to a caphaving an externally threaded stud (similar to the cap 460 shown in FIG.17). As the cap is rotated relative to base 538, and base 538 tightensagainst the internal surface of bone plate 18 and surrounding bone 28,the flexible struts are bowed to conform to the curvature of theinternal surface of the cranial vault.

[0099] Yet another embodiment of the strut base 638 is shown in FIGS. 21and 22, wherein there are multiple thin flexible struts 641 extendingfrom a central solid portion 639. Collar 646 projects upwardly from base638. Collar 646 is externally threaded to accept an internally threadedcap, and is also internally threaded to accept the externally threadedtip of the insertion instrument. The thin struts 641 are approximately 1mm thick, and may, for example, be 5-10 mm long.

[0100]FIG. 23 shows the base of FIGS. 21 and 22 wherein the flexiblestruts 641 of the base are bowed to conform to the curvature of theinternal skull as the curved cap 630 is tightened by screwing internallythreaded cap 630 on to externally threaded collar 646. The fasteninginstrument 102 is also shown attached to the fastener, with threaded tip128 screwed into the internal threads of collar 646 to hold base 638against the internal surface of the cranium while curved cap 630 isrotated to screw internally threaded cap 630 on to externally threadedcollar 646.

[0101] The flexible struts of the base provide more flexibility than asolid member. The struts are preferably made of a high densitypolyethylene. In the disclosed embodiment, each strut does not occupymore than about a 20-30° sector of the base. The disclosed struts arerectangular in shape, but they can alternatively taper, flare, or assumeother shapes that sufficiently reduce the solid areas of the base toincrease the flexibility of the base such that it conforms to thecurvature of the skull when the fastener is tightened to such an extentthat the bone in the skull is not fractured or damaged. For example, themodulus of flexibility of the base (or each individual strut) is 300,000to 700,000 psi.

[0102] The fastener instrument of the present invention is preferablymade of a metallic material that is suitable for sterile use in surgicalprocedures. Titanium or stainless steel are examples of such materials,particularly for the embodiments that do not flex. The fastener orinstrument may also be made of a durable plastic material. Plastic is aparticularly preferred material for embodiments of the invention thatuse flexible struts (such as the embodiments of FIGS. 18-23). Theinvention also includes a fastener, positioning instrument, andfastening instrument that has been sterilized, for example in anautoclave, or by irradiation (e.g. irradiation with ultravioletradiation), or by chemical sterilization (e.g. with disinfectants oranti-microbials).

[0103] As used in this specification, the term “conforms to a curvedmember” does not require identical curvature of the conforming memberand the surface to which it conforms. Substantial conformity of themembers is all that is required, and that includes surfaces between flatplanes and curves that precisely conforms to the curved member againstwhich the conforming number slats.

[0104] Having illustrated and described the principles of the inventionin several preferred embodiments, it should be apparent to those skilledin the art that the invention can be modified in arrangement and detailwithout departing from such principles.

What is claimed is:
 1. A fastener for holding adjacent members together,the fastener comprising: first and second fastening members, wherein atleast one of the fastening members has a conforming surface thatconforms to a curved member; a first connector on the first fasteningmember; and a second connector on the second fastening member forconnecting with the first connector, and holding the first and secondfastening members in a fixed relationship that holds the adjacentmembers together; wherein the adjacent members are bone plates.